As described in Japanese Patent Application Publication No. 2004-278370 (JP2004-278370A), the ratio of rotational speeds between counter-rotating first and second impellers of a counter-rotating axial-flow fan is usually fixed.
As disclosed in Japanese Patent Application Publication No. 02-238195 (JP02-238195A), when a counter-rotating axial-flow fan is used for forced air cooling, the rotational speed of a first motor for rotating a first impeller and that of a second motor for rotating a second impeller are separately controlled. Namely, the ratio of rotational speeds between the first and second impellers is changed according to varying ambient temperature. Specifically, in this disclosure, the rotational speed of the second impeller located downstream is set lower than that of the first impeller located upstream if it is necessary to increase a flow rate in order to reduce noise, and the second impeller located downstream is stopped if an airflow is sufficient.